[java] Polymorphism: Why use "List list = new ArrayList" instead of "ArrayList list = new ArrayList"?

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Why should the interface for a Java class be prefered?

When should I use

List<Object> list = new ArrayList<Object>();

ArrayList inherits from List, so if some features in ArrayList aren't in List, then I will have lost some of the features of ArrayList, right? And the compiler will notice an error when trying to access these methods?

This is also helpful when exposing a public interface. If you have a method like this,

public ArrayList getList();

Then you decide to change it to,

public LinkedList getList();

Anyone who was doing ArrayList list = yourClass.getList() will need to change their code. On the other hand, if you do,

public List getList();

Changing the implementation doesn't change anything for the users of your API.

I think @tsatiz's answer is mostly right (programming to an interface rather than an implementation). However, by programming to the interface you won't lose any functionality. Let me explain.

If you declare your variable as a List<type> list = new ArrayList<type> you do not actually lose any functionality of the ArrayList. All you need to do is to cast your list down to an ArrayList. Here's an example:

List<String> list = new ArrayList<String>();
((ArrayList<String>) list).ensureCapacity(19);

Ultimately I think tsatiz is correct as once you cast to an ArrayList you're no longer coding to an interface. However, it's still a good practice to initially code to an interface and, if it later becomes necessary, code to an implementation if you must.

Hope that helps!

This is called programming to interface. This will be helpful in case if you wish to move to some other implementation of List in the future. If you want some methods in ArrayList then you would need to program to the implementation that is ArrayList a = new ArrayList().

This enables you to write something like:

void doSomething() {
    List<String>list = new ArrayList<String>();
    //do something
}

Later on, you might want to change it to:

void doSomething() {
    List<String>list = new LinkedList<String>();
    //do something
}

without having to change the rest of the method.

However, if you want to use a CopyOnWriteArrayList for example, you would need to declare it as such, and not as a List if you wanted to use its extra methods (addIfAbsent for example):

void doSomething() {
    CopyOnWriteArrayList<String>list = new CopyOnWriteArrayList<String>();
    //do something, for example:
    list.addIfAbsent("abc");
}

I guess the core of your question is why to program to an interface, not to an implementation

Simply because an interface gives you more abstraction, and makes the code more flexible and resilient to changes, because you can use different implementations of the same interface(in this case you may want to change your List implementation to a linkedList instead of an ArrayList ) without changing its client.

In general you want to program against an interface. This allows you to exchange the implementation at any time. This is very useful especially when you get passed an implementation you don't know.

However, there are certain situations where you prefer to use the concrete implementation. For example when serialize in GWT.

I use that construction whenever I don't want to add complexity to the problem. It's just a list, no need to say what kind of List it is, as it doesn't matter to the problem. I often use Collection for most of my solutions, as, in the end, most of the times, for the rest of the software, what really matters is the content it holds, and I don't want to add new objects to the Collection.

Futhermore, you use that construction when you think that you may want to change the implemenation of list you are using. Let's say you were using the construction with an ArrayList, and your problem wasn't thread safe. Now, you want to make it thread safe, and for part of your solution, you change to use a Vector, for example. As for the other uses of that list won't matter if it's a AraryList or a Vector, just a List, no new modifications will be needed.